How does a cell know where to leave blood vessel to reach a site of infection?
How do cells move through the extracellular matrix?
Extravasation
Attaching to the endothelial lining
Migration of cell through capillary wall
Four steps:
Activation
Cytokines activate endothelial cells from Ab-Ag interaction or release by mast cells
Causes translocation of P-selectin vesicles and exocytosis to cell surface within seconds
Production of PAF (platelet activating factor) by endothelium
Trapping
P-selectin temporally binds to selectin receptor on surface of leukocyte
Hydrogen bond
Easily broken, causing cell rolling, binding more and more selectin and slowing cell down
PAF activates PAF receptor on leukocyte that causes conformational change in integrin on surface of leukocyte
Adhesion
Integrin of leukocyte binds to ICAM (intracellular adhesion molecule) of endothelial cell.
Leukocyte now tightly attached and cell thins and spreads out over capillary surface
Leukocyte adhesion deficiency:
Improperly produced integrin
Leukocytes cannot adhere and cannot effectively migrate out of blood vessel
Migration
Flattened cell reaches out with lamellipodia or pseudopodia and sticks down between cell, grabbing basement membrane and pull itself through
Leukocyte secretes metalloproteases to break junctional complex
Focal Contact
Transmembrane protein Integrin
Cytosolic side linked to cytoskeleton filaments of actin via linker proteins talin, vinculin, and alpha actinin
Exterior side attaches to laminin, collagen IV, tenacin, or fibronectin
Integrin
Transmembrane protein
Consists of α and β heterodimers
14 types of α subunits
8 types of β subunits
Different combinations of α and β heterodimers can adjust the binding affinity and binding specificity of intercellular interaction
Binding site is inbetween α and β heterodimers
α subunits bind to components of extracellular matrix
β subunits binds to only leukocytes
Requires activation in order to bind to ICAM
Phosphorylation of integrin causes integrins to dissociate from actin cortex and disperse
Acts in mitosis to stop integrin from binding fibronectin allowing cells to round up and undergo mitosis
Thought to play a role in cancer metastasis
Glanzmann’s disease
Normally, integrin needs to be activated to bind fibrinogen and participate in clotting
Must bind to collagen of thrombin first to activate integrin
Lack of β3 integrin causes causes excessive bleeding due to lack of clotting
Diapedesis
Process of migration through extracellular matrix to infection site
Second phase of cell motility
Occurs within connective tissue
Two types:
Slow Moving
Occurs in fibroblasts and growth cones of neurons
Fast Moving
occurs in leukocytes and macrophages
Can see cytoplasmic streaming
Four steps:
Extension of lamellipodia (podosomes)
Adhesion via attachments by focal contacts to adhesion molecules and collagen
Attaches cell to substrate
Gives cell point to pull against
Cytoplasmic flow forward
Cytoskeleton (actin) shifts forward
Cytoplasm goes from a gel to a sol state
Retraction with footprint
Trailing edge thins
Forms retraction fiber
Fibers snaps to leave a “footprint”
Possible Mechanisms
1st Possibility: Leading Edge
Actin filaments attached to cell membrane by myosin I
Profilin adds actin monomers to leading edge of actin, extending cytoplasm forward
2nd Possibility: Actin Filament Elongation
No profiling, actin monomers added to leading edge, extending cytoplasm
3rd Possibility: Mysoin Movement
Actin filaments added without profiling
Myosin moves within fluid mosaic, pushing actin filaments forward
Intrinsic to all three mechanisms is the concept that the nucleus and all other organelles are attached to the cytoskeleton and are moved forward with the framework
Cell Navigation
Chemotactic Factors
Calcium concentration gradient
Extracellular Matrix Order
Integrin is central to matrix organization
Orientation of cytoskeleton in cell orients the assembly of secreted extracellular matrix molecules in the vicinity
The oriented extracellular matrix reaches other cells and orients the cytoskeleton of those cells
Newly oriented cells now secrete an oriented matrix in their vicinity
Ordering of cytoskeletons now propagated by newly oriented cells
Organ Formation
Basal lamina and mesenchyme presence essential for salivary gland formation
Removal of basal lamina, mesenchyme results in non-differentiation
Endoderm, mesoderm, and basal lamina must interact to form functional mature gland
Reconstruction of Myoneural Junctions
Reconstruction of regenerating myoneural junctions dependent on basal lamina
Basal lamina surrounds muscle cell
Synaptic regions contain special forms of collagen IV and laminin
In injury, both skeletal muscle and nerve are destroyed, leaving just the basal lamina
Blocking formation of either skeletal muscle doesn’t prevent the formation of nerve or vice versa
Basal lamina contains information necessary for regeneration of skeletal muscle and nerve
Lamina
Lamina regulates neuronal outgrowth
Growth cones select specific molecules to migrate on
Fibronectin
Fibronectin promotes migration
Phosphorylation inactivates binding of Integrin
Promotes migration in mesenchyme
Incorporated in blood clots
Promotes migration of epithelial cells across wound
Promotes infiltration of fibroblasts to migrate into clot from surrounding connective tissue to repair gap and form scar tissue
Cell Motility
Dr. Robert Crissman, Ph.D.
Table of Contents
Importance of Cell Mobility
Mechanisms of Cell Adhesion
Junctional Attachments
Non-junctional
Stages of Cell Motility
Extravasation
Focal Contact
Integrin
Glanzmann’s disease
Diapedesis
Possible Mechanisms
Cell Navigation
Extracellular Matrix Order
Organ Formation
Reconstruction of Myoneural Junctions
Lamina
Fibronectin